Closed loop control of the current through an electrical device is well known. A commanded or desired level of current in the device is provided to a current driver and a feedback signal indicative of the actual current in the device is provided from some sensor in privity with the actual current. A controller makes adjustments to the driver control in response to the feedback signal in direction to minimize the difference between the actual and desired current.
Closed loop current controllers may vary in precision and flexibility depending on design constraints. An actual current feedback signal is most critical in control environments in which unpredictable variations in drive voltage or electrical load are expected. An automobile is an example of such an environment, as the demands on the supply voltage from the vehicle battery can vary unpredictably and may affect the regulated battery output voltage. Furthermore, the device being driven in an automobile may be in a harsh environment wherein temperature extremes, moisture and vibration can affect the electrical load of the device.
It is generally known that a desired level of current through a device may be provided by periodically applying a voltage of known magnitude across the device, wherein the periodicity, voltage magnitude, and load characteristics affect the current through the device. A fast acting gate, such as a conventional MOSFET driver may be used to periodically apply the voltage across the device, wherein the MOSFET is pulse width modulated at a duty cycle appropriate for the desired level of current through the device. The driver may be disposed between the supply voltage and the device, a high side driver, between the device and ground, a low side driver, or other, more complicated drive configurations may be provided.
One such configuration is the well-known full H-bridge, wherein a driver is situated above and below each terminal of the device. The advantages of full H-bridges, especially their efficiency and controllability, are well-documented. Pairs of drivers at opposite corners of the bridge conduct to form a path from the supply to ground through the load. One pair conducts to drive current through the device in a first direction and the second pair conducts to drive current through the device in a second direction. Typically, current control through the device via pulse width modulation is provided by allowing the driver between the supply and the device to conduct 100 percent of the time, and pulse width modulating the driver between the device and ground.
When the modulated driver is on, current passes from the supply through the steadily conducting driver, through the device, through the modulated driver, to ground. The current through the device, if it has any significant inductive load, will be charging up at this time. During the time in a cycle when the modulated driver is off, the current will typically be recirculated up to the supply via a flyback diode. The potential across the driven device, from the charging of its inductive load, will discharge according to a generally known schedule as it continues to drive current through the upper portion of the bridge.
Feedback is provided in these drive configurations by interposing a sense resistor in series with the drive circuitry, typically between the drive circuitry or the load and ground. The potential across the sense resistor is communicated to a current controller as an indication of the current through the load. When the modulated driver in a full H-bridge is not conducting, no current passes through the sense resistor, and it therefore cannot inform the controller of the drive current.
Pulse width modulated control of the driver requires an estimation of the driver enable time and driver disable time once per cycle. A common time to turn the modulated driver back on which is when recharging of the device is resumed, is when the current through the device is reduced a predetermined offset below the upper current value. The upper current value is the current level the device must be driven at before the modulated driver will be disabled, as described. The offset, which defines an amount of tolerated current ripple through the device, is typically a narrow range of acceptable driver currents around a desired drive current. The magnitude of the range depends mainly on the control precision required in the application.
The driver turn-off time is conventionally determined directly from the voltage drop across the conducting sense resistor, which may be compared directly to the commanded current level. However, once the driver is turned off, as discussed, no such feedback information is available to the controller. Accordingly, some means of estimation of the proper turn on time must be made.
One approach to estimation of turn on time is modelling the electrical load of the driven device, such as with a parallel combination of a capacitor and resistor. By charging the load and the modelled load contemporaneously, and by monitoring the discharge of the model while both are discharging, the appropriate time to resume charging may be estimated.
This approach is unacceptable in certain applications, such as the above-described automotive applications, in which the device being driven may be subject to a harsh environment which may affect the nature of its electrical load. For instance, if the device operates in an environment subject to significant temperature changes, the rate at which it charges and discharges can vary significantly, due to the temperature dependance of its electrical impedance. Such changes may not be easily reflected in the model thus reducing the accuracy of the model.
The effects of aging and contamination are also difficult to model, and may significantly affect the nature of the device. Accordingly, what is needed is a method and apparatus to accurately estimate the current through the load in a conventional driver configuration when the modulated driver is not conducting.